1. Field of the Invention
The present invention generally relates to polishing, planarization, plating and combinations thereof. More particularly, the invention relates to the monitoring and control of electro-processing, polishing and plating.
2. Description of the Related Art
Sub-quarter micron multi-level metallization is one of the key technologies for the next generation of ultra large-scale integration (ULSI). The multilevel interconnects that lie at the heart of this technology require planarization of interconnect features formed in high aspect ratio apertures, including contacts, vias, trenches and other features. Reliable formation of these interconnect features is very important to the success of ULSI and to the continued effort to increase circuit density and quality on individual wafers and die.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting, and dielectric materials are deposited on or removed from a surface of a wafer. Thin layers of conducting, semiconducting, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and electrochemical plating (ECP).
As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer may become non-planar across its surface and require planarization. An example of a non-planar process is the deposition of copper films with the ECP process in which the copper topography simply follows the already existing non-planar topography of the wafer surface, especially for lines wider than 10 microns. Planarizing a surface, or “polishing” a surface, is a process where material is removed from the surface of the wafer to form a generally even, planar surface. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials. Planarization is also useful in forming features on a wafer by removing excess deposited material used to fill the features and to provide an even surface for subsequent levels of metallization and processing.
Chemical Mechanical Planarization, or Chemical Mechanical Polishing (CMP), is a common technique used to planarize wafers. CMP utilizes a chemical composition, typically a slurry or other fluid medium, for selective removal of materials from wafers. In conventional CMP techniques, a wafer carrier or polishing head is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The carrier assembly provides a controllable pressure to the wafer, thereby pressing the wafer against the polishing pad. The pad is moved relative to the wafer by an external driving force. The CMP apparatus affects polishing or rubbing movements between the surface of the wafer and the polishing pad while dispersing a polishing composition to affect chemical activities and/or mechanical activities and consequential removal of materials from the surface of the wafer.
Another planarization technique is Electro Chemical Mechanical Polishing (ECMP). ECMP techniques remove conductive materials from a wafer surface by electrochemical dissolution while concurrently polishing the wafer with reduced mechanical abrasion compared to conventional CMP processes. The electrochemical dissolution is performed by applying a bias between a cathode and a wafer surface to remove conductive materials from the wafer surface into a surrounding electrolyte. Typically, the bias is applied by a ring of conductive contacts to the wafer surface in a wafer support device, such as a wafer carrier head. Mechanical abrasion is performed by positioning the wafer in contact with conventional polishing pads and providing relative motion there between.
An objective of polishing is to remove a predictable amount of material to achieve a desired profile. Accordingly, any polishing technique requires an endpoint detection to determine when the appropriate amount of material has been removed for various regions of the wafer. However, the progress of the polishing operation is not easily viewable because of the contact between the wafer and the pad.
In addition, variations in the polishing conditions impede an accurate determination of the polishing endpoint. Variations in the slurry/electrolyte composition, pad condition, relative speed between the pad and the wafer, and the load of the wafer on the pad, etc, cause variations in the material removal rate, which change the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be estimated merely as a function of polishing time.
One approach to predict the polishing endpoint is to remove the wafer from the polishing apparatus and measure the thickness of the remaining film on the wafer. Doing so periodically during polishing, the quantity of material being removed from the wafer may be determined. As such, a linear approximation of the material removal rate may be used to determine the polishing endpoint. However, this method is time consuming, and does not account for sudden changes in the removal rate that may occur between measurement intervals.
Several non-invasive methods of endpoint detection are known. One type of endpoint detection typically requires access to at least a portion of the wafer surface being polished, such as by sliding a portion of the wafer over the edge of the polishing pad or through a window in the pad, and simultaneously analyzing the exposed portion of the wafer. For example, where polishing is used to expose metal lines embedded in a dielectric layer, the overall or composite reflectivity of the surface being polished changes as the lines are exposed. By monitoring the reflectivity of the polished surface or the wavelength of light reflected from the surface, the exposure of the lines through the dielectric layer, and thus the polishing endpoint, can be detected. However, this method does not provide a way of determining the polishing endpoint unless an underlying layer is exposed during polishing. Additionally, this approach is somewhat erratic in predicting the polishing endpoint unless all of the underlying lines are simultaneously exposed. Furthermore, the detection apparatus is delicate and subject to frequent breakdown caused by the exposure of the measuring or detecting apparatus to the slurry or electrolytic fluid.
A second type of method for determining the polishing endpoint monitors various process parameters, and indicates an endpoint when one or more of the parameters abruptly change. For example, the coefficient of friction at the interface of the polishing pad and the wafer is a function of the surface condition of the wafer. Where an underlying material different from the film being polished is exposed, the coefficient of friction will change also. This affects the torque necessary to provide the desired polishing pad speed. By monitoring this change, the endpoint may be detected.
In an ideal system, where no parameter other than the wafer surface changes, process parameter endpoint detection is acceptable. However, as the wafer is being polished, the pad condition and the slurry/electrolyte composition at the pad-wafer interface also change. Such changes may mask the exposure of the underlying metal layer, or they may imitate an endpoint condition, leading to a premature stop of polishing.
Finally, ECMP presents a chemically, electrically and physically unique environment, with respect to conventional CMP. Thus, while the endpoint detection techniques (including those described above) exist for CMP, the techniques may not be readily extendible to ECMP. Even where the techniques are extendible to ECMP, doing so may require retrofitting existing processing systems with expensive equipment. A preferred approach would mitigate or avoid the challenges with retrofitting existing systems.
Therefore, there is a need for polishing control, and in particular there is a need for endpoint detection, which accurately and reliably determines when to cease polishing during electro-processing.